Uranium-rich accessory minerals in the peraluminous and perphosphorous Belvís de Monroy pluton (Iberian Variscan belt)

摘要: The strongly peraluminous, perphosphorous (<0.85 wt% P2O5) and low-Ca granites from the Belvis de Monroy pluton contain most U-rich monazite-(Ce) xenotime known in igneous rocks. Along with these accessory minerals, P-rich zircon occurs, reaching uncommon compositions particularly more fractionated units of this zoned pluton. Monazite displays a wide compositional variation UO2 (<23.13 wt%) ThO2 (<19.58 wt%), positively correlated Ca, Si, P, Y REE. Xenotime shows high content (2.37–13.34 wt%) parallel increases LREE, Ca Si. Zircon contains comparatively much lower (<1.53 wt%) but P2O5 (<14.91 wt%), Al2O3 (<6.96 wt%), FeO (<2.93 wt%) CaO (<2.24 wt%) contents. main mechanism incorporating large U Th amounts studied monazite is cheralite-type [(Th,U)4+ + Ca2+ = 2(Y,REE)3+] substitution. requires several coupled mechanisms to charge balance P substitution, resulting non-stoichiometric low analytical totals. Compositional variations phases indicate that substitution during crystal growth depend on availability non-formula elements. strong character crystallization, triggering progressive impoverishment LREE residual melts, consequently increasing extraordinarily xenotime. This marked contrast other peraluminous (I-type or P-poor S-type) granite series. melt inhibits uraninite so contributing for